In recent years, there have been numerous studies on optical sensors for analyzing molecules with light. These studies include; Surface Enhanced Raman Scattering for enhancing the sensitivity of Raman scattering by local field enhancement which occurs when a local plasmon is excited in a noble metal such as Au and Ag; and Surface Plasmon Resonance Spectroscopy using the properties in which a resonant frequency of a local plasmon is sensitive to the environmental permittivity.
When a molecular measurement is generally performed with an optical sensor, a liquid solution which contains a substance to be targeted is dropped on the surface of the sensor, and probe light is incident under the conditions (e.g. kind, intensity and incident angle of the light) according to a measurement method. The reflected or scattered light is received by a light detector and is analyzed to measure (identify) the substance. If the measurement sensitivity of these optical sensors is increased to single-molecule level, it is expected that the sensors can be applied in many fields such as medical fields, biotechnologies and environments.
Controlling the shape and alignment of noble metal nanoparticles is the key to enhancing the sensitivity of a sensor which is used for molecular measurement with light. Since surface charge which enhances the local field appears on the edges of nanoparticles, if the control can be accomplished properly, the measurement sensitivity will be improved dramatically. In addition, to make the sensor available as a practical product, the sensor is required to not only have high sensitivity but also to simultaneously have high reproducibility and to be inexpensive to manufacture.
Conventionally, many technologies aimed at obtaining a sensor which meets the previously described conditions have been developed and disclosed. For example, Non-Patent Document 1 discloses a nanosphere lithography method as a method for manufacturing a substrate for Surface Enhanced Raman scattering. With this method, it is possible to align triangular nanoparticles regularly so that their apexes are facing each other by a relatively simple procedure, and highly-sensitive molecular measurement can be therefore performed with this substrate. However, the local field enhancement with this technology has a limitation attributable to the triangular shape of the nanoparticles.
Additionally, as one of the technologies regarding Raman scattering measurement sensor, Patent Document 1 discloses a sensor and the method for making this sensor in which particles having the same diameter and dimension whose surfaces are coated with metal are repeatedly aligned so as to form periodic irregularity on the surface of a particle layer. This sensor has uniformity and high reproducibility, and can be manufactured easily and inexpensively. Nonetheless, because the shape of the particles is spherical, the measurement sensitivity is not yet sufficient.
Holography and electron lithography are other methods for controlling the shape and alignment of nanoparticles. With these methods, it is possible to align regularly nanometer-scale particles (see Patent Document 3 and Non-Patent Document 2 for examples).
Unfortunately, these methods are impractical as a method for manufacturing disposable sensors for molecular measurement because the products are too expensive.
Patent Document 4 also discloses a technology regarding a substrate for Surface Enhanced Raman Scattering: a substrate utilizing a needle-like column structure which is obliquely adhered to the substrate by obliquely depositing a metal to the substrate. With this method, it is possible to form nanometer-scale self-assembled elongated metal nanorods; however, since the needle-like columns are not aligned with their edges facing each other, the enhancing effect is limited.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2004-170334
[Patent Document 2] Unexamined Japanese Patent Publication No. 2002-372620
[Patent Document 3] U.S. Pat. No. 4,448,485
[Patent Document 4] U.S. Pat. No. 5,017,007
[Non-Patent Document 1] John C. Hulteen et al. “Nanosphere Lithography: Size-Tunable Silver Nanoparticle and Surface Cluster Arrays”, J. Phys. Chem. B, 103, 3854-3863(1999)
[Non-Patent Document 2] P. F. Liao et al. “Surface-enhanced raman scattering from microlithographic silver particle surfaces”, Chemical Physics Letters Volume 82, number 2, 1 Sep. 1981